Patent Document 1 indicated below discloses a container lid composed of a synthetic resin inside plug to be mounted on the mouth neck of a container, and a synthetic resin lid body to be mounted on the inside plug. The inside plug includes a circular closing wall, and a cylindrical mounting wall connected to the outer peripheral edge of the closing wall. The mounting wall is fitted to the outer peripheral surface of the mouth neck of the container, whereby the inside plug is mounted on the mouth neck of the container, and the closing wall closes the mouth neck of the container . The lid body includes a circular covering wall, and a fitting wall connected to the outer peripheral surface of the covering wall. An internal thread formed in the inner peripheral surface of the fitting wall is screwed to an external thread formed on the outer peripheral surface of the mounting wall of the inside plug, whereby the lid body is mounted on the inside plug, and the covering wall is located above the closing wall of the inside plug. In the closing wall of the inside plug, a breakable thin-walled line is formed which defines a comma-shaped removal region, and a key-shaped locked piece is formed which extends out upwardly from the removal region. On the lower surface of the covering wall of the lid body, a key-shaped locking piece is formed which is locked to the locked piece of the inside plug. With the lid body being mounted on the inside plug as required, the lid body is rotated counterclockwise as viewed from above (in a direction in which the lid body is detached from the inside plug) relative to the inside plug. As a result, a force is exerted on the removal region of the inside plug via the locking piece of the lid body and the locked piece of the inside plug. Thus, the breakable thin-walled line is broken, so that the removal region is removed from the closing wall, whereupon a pass-through opening is formed in the closing wall.